1. The Field of the Invention
This invention relates to apparatus and methods for monitoring, non-intrusively, the contents of a container containing fluid. The invention relates particularly, but not exclusively, to a method of non-intrusively monitoring the gaseous contents of a container in order, for example, to confirm the composition or purity of the gas within the container.
2. The Relevant Technology
It is often important to be able to monitor and confirm the composition or purity of gas contained within a container in order to monitor possible events, such as corrosion of the container, or to detect a leakage of the gas contained within the container or the leakage of a gas into the container. This may be particularly important when the gas or other contents of the container are toxic or in some other way harmful.
The invention therefore has particular application in, for example, the nuclear industry where the storage of radioactive substances requires continual or periodic monitoring of storage conditions to confirm continuous safe storage.
It may also be useful to non-intrusively monitor the contents of a container holding hazardous fluid or solid in the form of, for example, flammable, biological or pharmaceutical materials.
The invention may also be useful in monitoring the contents of containers in the vicinity of potentially hazardous processes such as in the operation of high voltage switch gear where gas within containers provides electrical insulation for the switchgear.
Known methods and apparatus for monitoring the gaseous contents of a container in order to confirm the composition of the gas within the container require penetration of the container in order to sample the gas contained within the container or in order to introduce a sensor into the container.
A problem with such known methods and apparatus is that because it is necessary to penetrate the container, there is a risk that leakages from the container occur around the area where penetration has taken place. Such systems, in seeking to establish that no leakage is occurring have to generate a potential site for leakage. This is technically undesirable and a potential problem with regulatory authorities.
Spent nuclear fuel is highly radioactive and it is necessary to appropriately deal with the fuel to ensure that the radioactive spent fuel does not contaminate the environment.
There is a requirement to be able to safely store spent nuclear fuel for an intermediate period known as xe2x80x9cinterim storagexe2x80x9d which period may be prolonged if required, pending a decision as to whether reprocessing or disposal of the fuel is to be undertaken. Spent fuel is typically stored within a sealed container during such storage.
Typically, a container suitable for interim storage of spent fuel comprises a canister made of pressure vessel grade steel within which the spent fuel is held. The canister incorporates radioactive shielding in its lid. Once the canister has been filled with spent fuel, it is fitted with a lid and welded. The final welding of the lid seals the fuel. The lid of the canister will typically have a double seal. Prior to final sealing of the lid, the canister is filled with helium so that the spent fuel is held in a sealed container in a helium atmosphere.
To provide further radiation shielding the canister is placed in a concrete storage cask which is also fitted with a lid. The canister may be positioned within a concrete cask such that there is a space between the canister and the cask. The cask has inlet ports at the bottom and outlet ports at the top so that air may flow within the concrete cask in order to cool the canister.
The concrete outer cask provides shielding for both gamma and neutron radiation and protection against external hazards.
It is desirable to be able to, from time to time, monitor the contents of the canister in order to ensure that no untoward reactions are occurring within the canister. Such checks would also indicate the continued integrity of the fuel cladding in the canister.
A known method of monitoring spent fuel within a sealed canister or dual purpose metal cask involves opening the sealed canister to examine the fuel and the atmosphere surrounding the fuel known as the cover gas within the canister.
A disadvantage with this known method is that there is a risk of contamination to the surroundings and the facilities required are extensive and expensive. In addition, it is neither easy nor practical to be able to continuously monitor the canister and contents thereof using such a method.
A second known method of monitoring spent fuel within a canister is through use of an installed penetration by which it is possible to attach instrumentation to measure the pressure or quality of the cover gas over the spent fuel or the seal interspace gas. Measurement of such gases will provide information relating to the chemical composition of the cover gas in the canister.
A disadvantage of this known method is that the presence of the penetration prejudices or degrades the integrity of the containment barrier of the canister thus providing a potential leak which could lead to radioactive contamination.
According to a first aspect of the present invention, there is provided a method for non-intrusively monitoring the contents of a sealed container comprising steps of:
transmitting an ultrasonic signal through a wall of the container into the contents of the container, receiving a signal from within the container, and analysing the received signal thereby deducing the composition of the contents of the container. This can thus be achieved without having to unseal the container.
According to a second aspect of the present invention there is provided apparatus for non-intrusively monitoring the contents of a sealed container the apparatus comprising:
transmitter means for transmitting an ultra-sonic signal through a wall of the container into the contents of the container;
receiving means for receiving a signal from within the container;
analysing means for analysing the received signal thereby deducing the composition of the contents of the container.
By means of the present invention it is possible to measure at intermittent intervals the quality of the atmosphere within a container.
The canister may be a substantially gas tight canister. The canister may be a metal canister, for instance of carbon steel or stainless steel.
The canister may be formed of a body and one or more lid elements. The one or more lid elements may be sealed to the body in use. Preferably a first lid is provided, together with a second outer lid. Preferably the first lid is received within the opening to the canister. The first and/or second lid may rest on one or more internal lips provided by the canister. The one or more lid elements may be welded to the body. The welds may provide a gas tight seal between a first lid and the canister and a second lid and the canister.
Preferably the canister has the general form of a right cylinder. Preferably the lids are provided on the top end of the canister, most preferably within the profile of the side wall of the canister, such that an end wall of the side wall is exposed.
Preferably the canister contains spent nuclear fuel rods or other irradiated nuclear material.
The canister may be provided with an internal gas pressure of greater than ambient, a positive pressure. The positive pressure may be at least 1.1 atmospheres, more preferably at least 1.2 atmospheres. Preferably the gas in the canister is substantially helium.
Preferably the canister is provided within a further container in use. The outer container may be a cask, for instance a concrete cask. Preferably the internal configuration of the outer container generally matches the outer configuration of the canister.
The outer container may be provided with a lid to seal the body of the container following insertion of the canister.
Preferably the outer container is provided with a supply of cooling gas to its interior. Preferably the cooling gas directly cools the outside of the canister. The cooling gas is preferably air. An inlet to the inside of the outer container and an outlet therefrom may be provided. Preferably the inlet and outlets are dog-legged.
The invention may be used to measure the fluid contents of a container and may therefore be used to monitor a gaseous or liquid content of a container. The invention may be used to measure the presence of one or more components of a gas. For instance, the presence of air in an helium atmosphere or the presence of xenon and/or krypton in an helium atmosphere may be measured. The invention may be used to measure the level of one or more components of a gas. For instance, the level of air in an helium atmosphere may be measured or the level of xenon and/or krypton in an helium atmosphere may be measured.
Advantageously, the method comprises the steps of measuring the sound velocity and/or attenuation of the transmitted signal and/or the reflected signal. The sound velocity and/or attenuation may be considered at more than one frequency of transmitted signal.
By measuring velocity and attenuation at least two different frequencies of transmitted signal, unknown quantities in the calculations/algorithms required to derive information regarding a composition of the contents are eliminated from the calculations. The method may measure the velocity and/or attenuation using transmission measurements. The method may measure the velocity and/or attenuation using reflected measurements. Conveniently, the method comprises the step of measuring velocity and attenuation of both a transmitted signal, such as a xe2x80x9cline of sightxe2x80x9d received signal, and a reflected signal.
The transmitted ultrasound signal may be provided by a transducer.
Preferably the ultrasound source and/or receiver therefore is removed from the canister between at least some of the tests. The transmitter and/or receiver may be removed whenever the time period between tests exceeds 1 hour, or more preferably 1 day.
Preferably a calibration and/or checking station is provided for the transmitter and/or receiver and/or accompanying electronics between at least some of the tests. Preferably such checks are made when tests are separated by more than 1 hour and more preferably by more than 1 day.
Conveniently, the source of the transmitted signal is positioned outside the container, and a receiver is positioned outside the container. The source of the signal and/or receiver are preferably mounted on the lid of the container. The source of the signal and/or receiver may be mounted on a side wall of the container, preferably on the top of the side wall. The source of the signal and/or receiver are preferably provided in a housing.
The housing may be mounted on the lid of the canister, for instance the outer lid or the inner lid.
The housing may be mounted on the outer surface of the outer lid, with a passage connecting the monitoring location in the housing to the body of gas within the canister, the passage being provided in a passage defining element which passes through the lid or lids, the cross-sectional profile of the passage defining element as it passes through at least a part of a lid being less than the cross-sectional profile of the housing, parallel to the lid. Preferably the cross-sectional profile is less throughout the elements passage through the outer lid, and if present the inner lid.
The housing may be mounted on the outer surface of the inner lid, with a passage connecting the monitoring location in the housing to the body of gas within the canister, the passage being provided in a passage defining element which passes through the inner lid, the cross-sectional profile of the passage defining element as it passes through at least a part of the inner lid being less than the cross-sectional profile of the housing, parallel to the lid. Preferably the cross-sectional profile is less throughout the elements passage through the inner lid. Preferably the cross-sectional profile of the housing as it passes through the outer lid is substantially the same as the housings cross-sectional profile outside the outer lid.
The housing may be mounted on the end wall of the side wall of the canister, most preferably wholly within the outline of the extension of that side wall. The housing may be welded to the canister. The housing may be formed of one or more different materials.
The received signal is preferably subjected to signal processing to extract the desired information. The signal processing may involve Fast Fourier Transform and/or chromatic based processing. The signal processing may involve the application of one or more Gaussian processors to the signal. The processors are preferably nonorthogonal. Preferably the processors cover the range of transmitted and/or received signal frequencies. Three processors may be applied. Preferably the processor outputs are further processed algorithmically. Preferably the algorithm results corresponding to the nominal energy content of the signal and/or the dominant frequency and/or the effective bandwidth, most preferably all three.
The signal may be represented as a point on a three dimensional plot defined by the nominal energy content of the signal, the dominant frequency and the effective bandwidth.
The condition within the container may be represented as a point on a three dimensional plot. The change in conditions may be represented as a deviation in one or more dimensions relative to that point. The extent of the deviation may represent the magnitude of the change in conditions. The direction of the deviation may represent the type of change in conditions.
Alternatively however the transmitter may be positioned within the container and may be activated by, for example, a signal transmitted from outside the container.
A transmitter suitable for positioning within the container may comprise, for example, a tuning fork or resonant cavity.
The canister will normally be filled with helium at a pressure of about 1xc2xc atmospheres at the time of sealing the canister. By means of the present invention, it is possible to confirm the continued presence of helium, the absence of atmospheric gases principally oxygen, the absence of fission product gases. It is also possible to discriminate between oxygen and fission product gases, so indicating the type of failure occurring.
The continued presence of helium, which is a highly mobile gas, will confirm that the canister is still satisfactorily sealed.
The absence of oxygen will confirm that corrosion of the external surface of the canister has been inhibited.
The absence of fission product gases will confirm that no deterioration of fuel integrity has taken place since the loading of the spent fuel into the canister.
By transmitting ultrasonic sound waves into the canister and receiving waves back from the canister (waves which have passed through the internal gas) the effects of variation in gas temperature can be allowed for, and the nature and amount of any foreign gas identified.
The internal gas at a monitoring location may be investigated. The monitoring location may be within the body of the canister. The monitoring location is preferably provided outside the body of the canister, but still sealed relative to the surrounding environment. Preferably the monitoring location is provided with a housing, most preferably the housing for the transmitter and/or receiver for the ultrasound. Preferably the monitoring location is provided in proximity to the outside of the canister lid.
The monitoring location is preferably connected to the internal body of gas in the canister via a bore or other passageway. The bore may be of circular cross-section. Preferably the bore includes one or more dog-legs. Preferably the bore passes from within the canister body to outside. The bore most preferably passes through the lid or lids of the canister. The bore may alternatively pass through the side wall of the canister. The bore may pass up through the side wall of the canister, towards the lid end of the canister. The bore may pass through the side wall of the canister, for instance to connect to an element externally provided on the canister and leading to the monitoring location.
The monitoring location is preferably in proximity to the transmitter and/or receiver. Ideally the monitoring location is provided between the transmitter and the receiver. The transmitter and receiver may be separated by a gap of between 0.5 and 20 cm or more preferably 3 and 8 cm.
The transmitter and/or receiver are preferably separated from the monitoring location by a thickness of material. The material thickness is preferably at least 5 mm and more preferably at least 10 mm or even 20 mm.
Preferably a volume of gas is provided in the housing on the distal side of the monitoring location relative to the body of the canister. Preferably the body of gas has a greater extent, perpendicular to the bore supplying it and/or parallel to the axis of the transmitter/receiver alignment, than the monitoring location itself. A disc shaped gas volume may be provided. In this way reduction of noise is facilitated.
It has previously been thought impossible to use sound or ultrasonic waves to determine the contents of a canister due to the fact that the temperature of the gas within the canister will affect the characteristics of reflected sound waves.
In addition, in order to be able to deduce the composition of the fluid within the container, the signal sound wave has to travel through a thick container wall without unacceptable signal loss. It had previously been thought that the attenuation of signals due to the thickness and nature of the canister material would be an insurmountable problem in using ultra-sonic signals to determine the contents of the container.
In addition, further problems associated with the method of the present invention include attenuation of the signal due to the impedance mismatching at changes of material. Further, a large amount of noise is generated by spurious internal reflections of the signal.
The physical properties of helium, oxygen and krypton/xenon (the principle fission product gases) in terms of atomic weight, molecular weight and bonding characteristics are sufficiently different to one another to allow reliable discrimination at the levels which would be expected within a canister containing spent fuel.
The present invention therefore provides a method where ultra-sound is transmitted through a container wall which may be metallic and may have considerable thickness, or alternatively may be a non-metallic material.
The ultra-sound may traverse through the gas or fluid within the container and then be received and detected through the container wall at a different point to that of which the ultrasound was transmitted into the canister wall initially. Alternatively, it may be received and detected at the same point at which it was transmitted through the canister wall following reflection of the signal.
The passage of the signal through the gas atmosphere or fluid will have modified the signal signature so by comparing the transmitted and received signals the composition of the gas or fluid may be inferred
In particular, the speed of sound, its attenuation and its frequency may be modified by differences in gas or fluid composition.
Alternatively, the resonance frequencies of the container and internal structures will be altered by the contained gas or fluid composition and this change may be used to infer the composition.
The signal may be produced by means positioned outside the canister for example a transmitter.
Alternatively, the signal may be produced from transmission means contained within the canister. Such transmission means may be in the form of, for example, a tuning fork or resonant cavity which may be activated from outside the container.